The present invention relates to a unique door assembly for microwave ovens. In microwave heating appliances, the nature of the heating phenomenon is that of stressing certain of the molecules of the product to be heated by using an electromagnetic field, commonly in the heating frequency range of 2,450 MHz. One of the more serious problems with such microwave oven devices has been concern about radiation leakage and the resulting possibility of operator injury. As a result, the current Bureau of Radiological Health standards require that microwave ovens allow a leakage of no more than 1 milliwatt per square centimeter at the factory and 5 milliwatts per square centimeter in the hands of the ultimate user.
The primary area of such radiation leakage is the periphery of the oven door and for this reason a number of door designs have been developed to limit this leakage. Many oven doors use a choke seal to limit radiation leakage. Such a seal commonly comprises a cavity circumscribing the internal edge of the oven door. The choke seal cavity dimensions are specified in terms of fractions of the wavelength of the electromagnetic radiation to be suppressed. This type of seal is disclosed in U.S. Pat. Nos. 3,809,843; 3,678,238; 3,502,839; and 3,511,959.
Choke seals are somewhat disadvantageous in that they are highly frequency selective. Since the dimensional configuration of such a seal cavity is dependent on wave length, this type of seal generally will not be effective to suppress the harmonic frequencies which are commonly present. One approach taken to suppress such harmonics is to provide more complicated choke cavity structure. A microwave choke seal arrangement for suppression of both fundamental and second harmonic frequencies is disclosed in U.S. Pat. No. 3,668,357.
Another type of seal used to suppress leakage of microwave radiation is a capacitive seal, such as shown in U.S. Pat. No. 3,808,391. The seal plate disclosed therein is a thin metallic plate which covers the oven cavity and presses firmly against the edges of the cavity. The surface of the plate has a thin coating of a suitable dielectric organosol. Surrounding the capacitive seal is a conductive sealing ring which acts as a secondary seal. Since the seal plate necessarily must be allowed to flex to a certain degree, the secondary seal must not be positioned so as to interfere with flexure of the capacitive seal. This dimensional stability requirement plus the need for an extremely rugged door resulted in a relatively expensive door construction.